diff --git a/content_builder.go b/content_builder.go
index 312290b..7e5b005 100644
--- a/content_builder.go
+++ b/content_builder.go
@@ -24,8 +24,22 @@ func (b *contentBuilder) build() (string, error) {
 		return "", errors.New("contentBuilder.build(): protoFile is nil")
 	}
 
-	if len(b.file.GetMessageType()) != 1 {
-		return "", errors.New(b.file.GetName() + ": only one top-level type may be defined in a file (see https://cloud.google.com/pubsub/docs/schemas#schema_types). use nested types or imports (see https://developers.google.com/protocol-buffers/docs/proto)")
+	messageTopLevel := []*descriptorpb.DescriptorProto{}
+	messageTypes := b.file.GetMessageType()
+
+	// If the file contains multiple messages
+	if len(messageTypes) > 1 {
+		// Then, if there's no top-level message specified using the parameter
+		if b.messageTopLevel == "" {
+			return "", errors.New(b.file.GetName() + ": contains multiple top-level messages and no top-level message is identified, use --pubsub-schema_opt=top-level-message={message} to specify one top-level message.")
+		}
+
+		// Because there are multiple top-level messages, lookup the one defined by the top-level-message parameter
+		for _, messageType := range messageTypes {
+			if *messageType.Name == b.messageTopLevel {
+				messageTopLevel = append(messageTopLevel, messageType)
+			}
+		}
 	}
 
 	fmt.Fprintf(b.output, "// Code generated by protoc-gen-pubsub-schema. DO NOT EDIT.\n")
@@ -34,7 +48,7 @@ func (b *contentBuilder) build() (string, error) {
 	fmt.Fprintf(b.output, "// source: %s\n\n", b.file.GetName())
 	fmt.Fprintf(b.output, "syntax = \"%s\";\n\n", b.schemaSyntax)
 	fmt.Fprintf(b.output, "package %s;\n", b.file.GetPackage())
-	b.buildMessages(0, b.file.GetMessageType())
+	b.buildMessages(0, messageTopLevel)
 	b.buildEnums(0, b.file.GetEnumType())
 	return b.output.String(), nil
 }
diff --git a/response_builder.go b/response_builder.go
index 19ee1aa..e2768c0 100644
--- a/response_builder.go
+++ b/response_builder.go
@@ -2,16 +2,33 @@ package main
 
 import (
 	"errors"
+	"fmt"
+	"regexp"
 	"strings"
 
 	"google.golang.org/protobuf/types/descriptorpb"
 	"google.golang.org/protobuf/types/pluginpb"
 )
 
+const (
+	// Per Buf's definition of identifier
+	// https://protobuf.com/docs/language-spec#identifiers-and-keywords
+	identifier string = `[A-Za-z_]([A-Za-z_]|[0-9])*`
+
+	// Parse parameter of the form top-level-message={message}
+	keyTopLevelMessage string = "top-level-message"
+)
+
+var (
+	// RegEx to extract Message name from top-level-message parameter
+	regexTopLevelMessage string = fmt.Sprintf("%s=(%s)", keyTopLevelMessage, identifier)
+)
+
 type responseBuilder struct {
 	request         *pluginpb.CodeGeneratorRequest
 	schemaSyntax    string
 	messageEncoding string
+	messageTopLevel string
 	protoFiles      map[string]*descriptorpb.FileDescriptorProto
 	messageTypes    map[string]*descriptorpb.DescriptorProto
 	enums           map[string]*descriptorpb.EnumDescriptorProto
@@ -22,10 +39,12 @@ func newResponseBuilder(request *pluginpb.CodeGeneratorRequest) (*responseBuilde
 	if request == nil {
 		return nil, errors.New("newResponseBuilder(request *pluginpb.CodeGeneratorRequest): request is nil")
 	}
+
 	builder := &responseBuilder{
 		request,
 		getSyntax(request),
 		getEncoding(request),
+		getTopLevelMessage(request),
 		make(map[string]*descriptorpb.FileDescriptorProto),
 		make(map[string]*descriptorpb.DescriptorProto),
 		make(map[string]*descriptorpb.EnumDescriptorProto),
@@ -50,6 +69,27 @@ func getEncoding(request *pluginpb.CodeGeneratorRequest) string {
 	return "binary"
 }
 
+func getTopLevelMessage(request *pluginpb.CodeGeneratorRequest) string {
+	parameter := request.GetParameter()
+
+	// For consistency with getSyntax and getEncoding, check whether parameter contains key
+	if strings.Contains(parameter, fmt.Sprintf("%s=", keyTopLevelMessage)) {
+		// If the parameter contains the key, use a more expensive regex to extract the value
+		re := regexp.MustCompile(regexTopLevelMessage)
+		messages := re.FindAllStringSubmatch(parameter, -1)
+
+		// Expect single occurrence (not top-level-message=Foo,top-level-message=Bar)
+		if len(messages) == 1 {
+			// Don't return the entire substring ([0]), i.e. top-level-message=message
+			// Only return the value ([1]) i.e. message
+			return messages[0][1]
+		}
+	}
+
+	// Otherwise unable to determine top-level messsage name
+	return ""
+}
+
 func (b *responseBuilder) initProtoFiles() {
 	for _, protoFile := range b.request.GetProtoFile() {
 		b.protoFiles[protoFile.GetName()] = protoFile
diff --git a/response_builder_test.go b/response_builder_test.go
new file mode 100644
index 0000000..521aa35
--- /dev/null
+++ b/response_builder_test.go
@@ -0,0 +1,51 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+
+	"google.golang.org/protobuf/types/pluginpb"
+)
+
+func Test_getTopLevelMessage(t *testing.T) {
+	tests := []struct {
+		name      string
+		parameter string
+		want      string
+	}{
+		{
+			name:      "empty",
+			parameter: "",
+			want:      "",
+		},
+		{
+			// Return value on valid value
+			name:      "single valid value",
+			parameter: fmt.Sprintf("blah,blah,%s=Foo,blah,blah", keyTopLevelMessage),
+			want:      "Foo",
+		},
+		{
+			// Return empty string on invalid value
+			name:      "single invalid value",
+			parameter: "blah,blah,root-message=9,blah,blah",
+			want:      "",
+		},
+		{
+			// Return empty string for multiple values
+			name:      "multiple values",
+			parameter: fmt.Sprintf("blah,blah,%s=Foo,blah,%s=Bar,blah", keyTopLevelMessage, keyTopLevelMessage),
+			want:      "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			request := &pluginpb.CodeGeneratorRequest{
+				Parameter: &tt.parameter,
+			}
+			if got := getTopLevelMessage(request); got != tt.want {
+				t.Errorf("getTopLevelMessage() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}